This invention relates generally to high pressure mechanical seal assemblies constructed and used with pumps to prevent uncontrolled leakage of high pressure fluid along a rotating shaft and, more particularly, to mechanical seal assemblies using hard, but relatively brittle, seal face materials that may chip or shatter during a malfunction of the pump or other error.
High pressure mechanical seal assemblies are used with nuclear reactor coolant pumps, boiler recirculating pumps, boiler feed pumps and pipeline pumps to meet the requirements for extreme and widely changing conditions of pressures and temperatures encountered in these uses. Examples of such seal assemblies are disclosed in U.S. Pat. No. 4,586,719 to Marsi et al. which issued May 6, 1986 and U.S. Pat. No. 5,076,589 to Marsi which issued Dec. 31, 1991.
A mechanical seal assembly usually includes the combination of a rotatable seal ring connected to a rotatable shaft for rotation with the shaft and a non-rotatable or stationary seal ring connected to a flange of a housing. Each seal ring has a radially disposed seal face and the seal faces oppose each other. There is usually a film of fluid between them, providing lubrication for the rotation of one of the faces. In many seal assemblies, one or more coil springs urge one of the rings toward the other, so that in reality, one or both of the seal rings are capable of limited axial movement, even though they are commonly referred to as "stationary" or "rotatable." Multiple stage seal assemblies comprising a plurality of seal assemblies are known in the art.
The stationary and rotatable seal rings are typically made of different materials. In some applications, the stationary seal face is carbon graphite and the rotating seal face is a harder material such as tungsten carbide, silicon carbide or the like. Silicon carbide is sometimes preferred over tungsten carbide because it is a harder material having a longer life span. It also causes less wear to the carbon graphite stationary seal face and does not corrode as much as tungsten carbide.
One drawback to the use of silicon carbide is that it has a relatively low impact strength, causing it sometimes to chip or shatter if a sudden load is applied to it. Thus, in the case of mechanical seal assemblies used for nuclear service, special care must be given to properly enclose a seal ring made of silicon carbide. This is because a shattered seal ring could have serious consequences if the broken pieces were permitted to escape from the seal assembly into the reactor system possibly causing damage, plugging or jamming of components.
A previous method used to secure the rotatable seal ring to the shaft was by shrink fitting a cylindrical member to the outside of the seal ring. The shrink fit member also enclosed the seal ring, preventing the escape of any chipped pieces of the ring. Shrink fits, however, transmitted radial contact forces to the seal rings, which could adversely affect the sealing properties of the assembly. Careful engineering was thus required to compensate for the shrink-fit forces and thus limit distortion of the seal face.
More recently, less expensive methods of securing the rotating seal ring to the shaft without using shrink fits have been developed. However, these new methods do not fully enclose the rotatable seal ring and therefore have not been used with silicon carbide rings. Accordingly, there still is a need for enshrouding a mechanical seal assembly having a harder, but possibly brittle, seal ring in such a way as to not influence the seal ring pressure and thermal deflection, but which also safely contains the pieces of the ring so that they do not scatter throughout the pump and piping should the ring shatter in the case of a catastrophic overload due to a fault condition such as a malfunction of the pump or other error. This invention satisfies that need.